R600: Check if a sextload should be used for parameter loads.

Through some oddity where truncate (sextload x) isn't folded into
an anyextload for vectors, the sextload remains if the
vector isn't immediately scalarized. This keeps the expected
zextload instructions in the kernel-args test when small type
vectors aren't scalarized.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206070 91177308-0d34-0410-b5e6-96231b3b80d8